The evolutionary history of Drosophila transposable elements was studied. More than ten different families of transposable elements identified in D. melanogaster were found also in related Drosophila species. The single exception was the p element that was found only in stocks of D. melanogaster recently collected from wild populations. It appears that most transposable elements are stable components of the host genome. The single exception among those studies is the P element that may have recently invaded. Theoretical studies of the extinction process of a transposable element family in a Mendelian host population were carried out. The impact of mutation within the transposable element to a defective copy was examined. Although values of various parameters can have large effects on the extinction time, it is generally true that mutation to defective elements accelerates the extinction process. This provides a scenario for the evolution of the P element in Drosophila melanogaster that rearranges to putative defective copies at a high rate.